Acoustic signal generators have many applications in modern technology. One example of such applications is a Personal Acoustic Alarm System described in copending patent application Ser. No. 348,245, filed on Feb. 12, 1982, now U.S. Pat. No. 4,473,821, the disclosure of which is incorporated herein by reference.
Applications for acoustic signal generators such as the above-mentioned Alarm System require a variety of output signal frequencies. For example, the system disclosed by the referenced patent application requires a plurality of different output signal frequencies in the audio range.
Many acoustic signal generators utilize a Helmholtz resonator having a piezoelectric generator disc associated therewith. These acoustic signal generators include circular metallic discs having a thin layer of piezoelectric material bonded to one side thereof. When an alternating voltage is applied to the piezoelectric material, the resulting mechanical forces distort the disc, causing it to oscillate as a diaphragm. If the disc is driven at a natural fundamental frequency and the Helmholtz resonator associated with that disc is tuned to the same frequency, a high acoustic energy output will occur. Each acoustic signal generator is unique to the disc dimensions and to the signal frequency of interest. Those generators described in the prior art are integral, molded units wherein the casing and the Helmholtz resonators are formed to be a single integral unit. These units, once formed, are intended for use at only one signal frequency and are not intended to be modifiable to produce high-energy signals at frequencies other than the one signal frequency associated with the generator as orignally manufactured and sold. Therefore, if a multiplicity of signal frequencies were of interest to the user, that user had to purchase and have available a corresponding multiplicity of completely separate generator units. This situation is both costly and onerous for the user.
Another disadvantage of known acoustic signal generators is the lack of acoustic signal volume produced. Even though the acoustic generator disc will vibrate symmetrically in two directions, currently known acoustic signal generators have only one Helmholtz resonator per unit. Often only one side of the disc is exposed to the Helmholtz resonator while the other side of the disc is exposed to a casing wall. In such a configuration, those sonic waves directed from the disc toward the casing wall not only are lost, but also may, upon reflection from the wall, actually interfere with the sonic waves directed from the disc toward the Helmholtz resonator. Furthermore, because only one Helmholtz resonator is associated with such signal generator unit, a unit can produce an output signal at only one frequency and in only one direction. Thus, these units must be carefully oriented to generate a signal in the desired direction.
It is apparent from the foregoing that currently known acoustic signal generators are not efficient and cannot make maximum use of the signal producing capability of an acoustic generator disc. For acoustic alarm applications, it is imperative that the acoustic transmitter produce an acoustic signal having as high a volume as possible.